How long will my money last in retirement? A real answer, not a magic number

The question Quora keeps asking

"How long will my money last in retirement?" comes up over and over, and the honest answer annoys people: it depends, and anyone who gives you one number is guessing. Two retirees can have identical balances, identical withdrawals, and get wildly different outcomes, because the order of the bad years matters as much as the average.

The math that actually works

The tool that handles this is a Monte Carlo simulation. Instead of one smooth 6% line, it runs thousands of possible futures, each with returns that bounce around a mean with realistic volatility, with your withdrawals bumped up for inflation each year. Then it counts how many of those futures still have money at the end. That percentage is your survival odds. This is what the Monte Carlo simulator here does, and it runs entirely in your browser, so you can poke at it without a spreadsheet.

What the percentage really tells you

If you get 80% survival, that does not mean you have an 80% safe plan. It means two out of every ten plausible histories run you out of money, and by definition you will not know which history you are in until late in the game. Most people are fine with a number like 90% plus. Some want 99%. The tool lets you see how deep the hole gets in the worst case, which is the more useful output anyway.

The levers that actually move the odds

Three knobs matter: the starting withdrawal rate, the volatility you assume, and how long the horizon runs. Drop from 4% to 3.5% and survival jumps noticeably, as our earlier 4% rule post shows with actual numbers. That half a percent is the single most expensive decision most people never get shown.

Stop asking for one number

Run the simulator with your real balance and your real monthly expenses, look at the 10th percentile outcome, and then decide whether to adjust spending, work longer, or shoulder the risk. That is the whole job. A range you can plan around beats one number that is secretly a guess.